1. Field of the Invention
The present invention relates to a silicone resin composition. More specifically, the present invention relates to a silicone resin composition having excellent transparency and a high refractive index, a method for producing the silicone resin composition, a sheet-like formed article made of the composition, and a photosemiconductor device encapsulated with the composition.
2. Discussion of the Related Art
Since silicone resins have excellent heat resistance and light fastness, as compared to materials made of organic polymers, there is an increasing use in the field of electronic materials that necessitate high durability. Among them, in a photosemiconductor encapsulating material having high brightness, a film for solar energy having strict operable environments and requiring durability, lens for which operable conditions are strict, or the like, silicone resins are widely used.
It is known that a disadvantage of a silicone resin is in a low refractive index. A general silicone resin has a refractive index of 1.4 or so, which is lower than a widely used material made of an organic polymer by 0.1 or so. A silicone resin having a low refractive index as mentioned above is desired to have the improvement in light-extracting efficiency, from the viewpoint of conservation of energy in, for example, a photosemiconductor encapsulating material having high brightness. Therefore, there is an urgent matter of improving a refractive index.
On the other hand, as a means of improving a refractive index of a silicon resin per se, Japanese Patent Laid-Open No. 2005-76003 reports a method of introducing a bulky aromatic substituent into a silicone resin. According to the publication, a refractive index can be increased from 1.4 to slightly over 1.5 by introducing a phenyl group having a high refractive index as a substituent for a silicon atom.
In addition, a method for adjusting a refractive index of a resin by dispersing a high-refractive index metal oxide, such as titanium oxide, zirconium oxide or barium titanate, in the resin is studied. For example, it is said that a material having an enhanced transmittance of visible light and excellent transparency can be provided by forming a metal oxide into fine particles. However, in general, it is difficult to disperse hydrophilic metal oxide particles in a hydrophobic and water-repellent silicone resin, and in particular, it is difficult to apply those particles to applications that require transparency. Even if the metal oxide particles are dispersed with a disperser or the like, although a thin film of 10 μm or so can relatively keep transparency, transparency is drastically lowered when a thickness of a film reaches that thickness or more.
In view of the above, Japanese Patent Laid-Open No. 2006-131547 discloses fine metal oxide particles dispersant formed by copolymerizing an acrylic site having affinity to titanium oxide particles with a silicone site having affinity to a silicone resin, in order to disperse the particles in the silicone resin. In addition, Japanese Patent Laid-Open No. Hei 9-208438 reports a method of dispersing titanium oxide by utilizing a silicic acid derivative which works as a dispersant.
On the other hand, in particles prepared on the basis of a sol-gel method or the like, the size of the primary particles can be generally controlled to a size of several nanometers to several dozen nanometers or so, so that there is an advantage of excellent monodisperse property. In a sol-gel method, since a reaction is usually carried out in a water-alcohol system, if the particles obtained are to be dispersed in the resin, a dispersion medium of the particles must be replaced with a solvent for dissolving the resin. However, even if the solvents are tried to be simply replaced, the affinity of the surface of the fine particles and the solvent is low, so that the particles are more likely to be aggregated in many cases. In view of the above, a method of carrying out a solvent substitution of particles prepared by a sol-gel method after treating the particles with a surface treatment agent such as a silane coupling agent is disclosed (see Japanese Patent Laid-Open Nos. 2000-63119, 2006-83033, and 2007-119617).